Better handle wrong user error after oauth refresh

[TheOneRing]

Display a proper error instead of an assertion.

[HanaGemela]

Steps to recreate

1. Connect to an oc10 server with oauth2 as Einstein
2. Log out in the client
3. Start login process in the client
4. Open the browser
5. Log in with Catherine

Test also with ocis

Expected result: Nice error that Logged in with a wrong user

[individual-it]

With ownCloud 10

after logging in with the wrong user in oc10, the site shows this error screen 👍

but after logging in again with the correct user the client does not work 👎

when using the "retry" button and using the correct user again the client stays in the "OpenBrowser" window 👎

The only way is to "Log out" and "Log in" again

logs: ownCloud.log

[individual-it]

with ocis

after logging in with the wrong user in ocis the focus changes to the client and this screen is shown 👍

clicking on "Retry" brings the user back to

"OpenBrowser" brings the user to this screen

clicking on "use an other account", logging in with the correct one and allow the connection, shows a "success window" in the browser but the client does not recognize it

The only way is to "Log out" and "Log in" again

logs: ownCloud.log

[fmoc]

Reproducible. We should open a new issue.